A large number of commercial products such as, for example, foodstuffs, cleaning products, and pharmaceuticals are dependent on the formation of specific molecular and macromolecular structures. The final product structure is responsible for its appearance, functionality, stability, compatibility with other materials or processes, and toxicology. Each particular structure is built by the mixing and inclusion of specific chemical or particulate components under controlled chemical, physical and environmental conditions. The control is required because many components used in forming these structured materials are required to be in a particular phase or state when another component is added or a particular part of the process applied. When a material undergoes a change in phase or state (e.g. solid to liquid, sol to gel, helix-coil, glass to rubber, crystalline to amorphous) the point at which this occurs is called the ‘phase or state transition’. The transition for a material will be determined by variables such as temperature, pressure, presence of solvent, ionic and small solute environment and concentration of the material.
Temperature is one of the key factors dictating the phase or state of many of the components commonly used on the aforementioned structured products. By way of example, heavy fats and waxes that are commonly used as polishes, emollients, surfactants and lubricants in formulation undergo a temperature dependant phase change from solid to liquid oil on heating due to disordering of small crystallites. This specific type of phase transition can be referred to as a ‘melt’ or ‘crystallisation’ dependant on the thermal direction of the process. Another predominantly thermal transition common in structured materials is the ‘helix-coil transition’ and this exists for a number of hydrocolloids such as Gellan Gum, Xanthan Gum and Gelatine. These biopolymers exist in their low energy state as hydrogen bonded double helices due to the specific linkage geometries between their constituent monosaccharides or amino acids. When heated, the energy input disrupts the hydrogen bonds and increases molecular motion allowing the helices to unwind and exist as free single polymers. This is the helix-coil transition. On cooling the helix-helix pairs reform, with each macromolecule pairing with one or more partners forming a cross-linked network. This ability to form network structures makes these polymers useful as viscosifiers, gelling agents and suspending agents.
Another set of transitions of importance in forming structured materials are those mediated by ionic bonding. Charged polymers such as pectins, alginate, carrageenan and low acyl Gellan are sensitive to metal cations, particularly positively charged divalent ions such as calcium. The ions bond with negatively charged sites on the polymers forming runs of crosslink between polymers called ‘junction zones’. The formation of junction zones leads to an increase in viscosity or gelation by the formation of a partial or fully networked structure. Below a critical ion concentration the junction zones cannot form stable cross-links and the system may be on the sol side of the gel-sol transition.
From the phase or state transition examples given above it is evident that multi-component structured materials formed from mixtures of these systems will often have constraints placed upon the formulation, process, or possible end product. This is due to undesirable behaviours, state or phase changes occurring in one or more component. An example of such unwanted behaviour may be in trying to form an emulsion from a long chain lipid requiring a relatively high temperature melt (e.g. 70° C.), but needing to be dispersed in a viscosified fluid phase that is not stable above 40° C. Introduction of the melted lipid into the cooler fluid will result in a ‘crash-cooling’ event whereby the lipid will rapidly pass through its crystallisation transition forming very small crystallites and rapidly coalescing forming irregular solid aggregates rather than a fine droplet dispersion for an emulsion. Use of high shear mixing equipment such as high pressure screen emulsifiers would not be advantageous in such a scenario due to clogging of the screen by the coalesced lipid. Also the high shear environment of the homogeniser would possibly disrupt the viscosifying structure in the fluid phase.
Other negative behaviours which may occur during the mixing and entrainment of seemingly incompatible products may include coalescence, precipitation, phase inversion, incorrect partitioning of solutes or ionic species between phases or components, phase separation, and inhomogeneity.
The following example is given as an illustration of the problems associated with mixing certain materials. In this example, Material 1 has a defined temperature T (or temperature range for some compound materials such as polymers) at which a phase change occurs.
T may be a temperature at which the material goes from a solid to a liquid phase, or it may be a temperature at which the helix coil transition occurs: above this temperature the helix coils unwind, below they form a cross-linked network. T may also be the temperature at which chemical bonds or cross-links mediated by charge interactions are broken or formed. For example calcium ion mediated bonds in the formation of low methoxy pectin gels.
Normally, in order to mix material 1 with a second material (“material 2”) material 2 must also be at a temperature above T, i.e. T2≧T. This may be disadvantageous simply because to raise material 2 to this temperature requires a large amount of energy. However it also may be disadvantageous because at this temperature material 2 has also passed through a phase transition, but in this case it means that material 2 is in an undesirable phase or state (e.g. the biopolymer Xanthan gum imparts specific rheological properties to a fluid in its low temperature ordered state, allowing it to both flow and act as a suspensor. At temperatures above its helix-coil transition temperature these properties are lost). Furthermore, even if the two materials are mixed at a temperature above T, the nature of the mixture so produced may mean that it then has to be cooled under very controlled conditions (and possibly over a long time) so as to maintained the desired mixture structure. This can be undesirable for cost and energy reasons. However, conventionally, if material 2 is not heated to above T then mixing the two materials is impossible. For example, if T2≦T and the phase change is a melt condition, then material 1 will instantly solidify when it meets material 2. This causes a “crash cooling” event whereby the lipid will rapidly pass through its crystallisation transition forming very small crystallites and rapidly coalescing to form irregular solid aggregates.
Another example scenario would be trying to mix fluid 1 into fluid 2 where fluid 1 undergoes a phase change in the presence of a critical ionic concentration C or pH present in fluid 2 (e.g. a dispersion of low ionic strength alginate, at gelling concentration, introduced into a fluid containing calcium ions above C). The alginate would rapidly form heterogeneous gelled particulates under conventional mixing. In this example temperature may also play a role in the ability and type of mixing and structure formed on introducing fluid 1 to fluid 2 by conventional mixing methods.